In conventional position-measuring devices for detecting the position of two objects movable relative to each other, usually the position of a scanning unit relative to a measuring standard having a measuring graduation disposed on it is determined along the graduation direction of the measuring standard, the graduation direction corresponding to the measuring direction; in this case, the scanning unit and the measuring standard are each joined to one of the two movable objects. In conventional devices, the so-called sensitivity vector of the position-measuring device, which denotes the specific effective measuring direction, is usually oriented parallel to the surface of the measuring standard.
In addition, certain conventional position-measuring devices have a sensitivity vector oriented obliquely to the surface of a measuring standard having a reflective measuring graduation. For example, reference is made to European Published Patent Application No. 1 762 828, which is expressly incorporated herein in its entirety by reference thereto. In such a position-measuring device, the inclined orientation of the sensitivity vector is ensured by an asymmetrical formation of the interferential scanning beam path. In correspondent scanning beam paths, an incoming beam of rays is split into at least two partial beams of rays that are ultimately brought to interfering superposition. With the aid of such position-measuring devices, it is possible to acquire position information with regard to a relative movement of the scanning unit and measuring standard along a lateral measuring or shift direction and along a vertical measuring or shift direction. That is, with the aid of a position-measuring device of this type, position changes are able to be detected along two translatory degrees of freedom of movement. In such a position-measuring device, the path lengths of the interfering partial beams of rays are usually equal only at a nominal scanning distance between the scanning unit and measuring standard. If the measuring standard or the scanning unit is moved out of this nominal scanning distance, different path lengths in the partial beams of rays reaching interference then result. Consequently, a possible change in the wavelength of the light source used influences the phase of the interfering partial beams of rays, and therefore also the position information ascertained. For that reason, the scanning optical systems of position-measuring devices of this type are described as chromatic. Therefore, the light source used in them must exhibit sufficient coherence length and an extremely low phase jitter. In order to ensure this, a complex stabilization of such a light source is necessary, making it correspondingly costly.